Ruby

What it actually is

Corundum, Al2O3, trigonal, hardness 9, coloured red by chromium substituting for aluminium. Every other colour of corundum is called sapphire.

Where pink becomes red enough to be called ruby is a trade convention rather than a mineralogical fact, and different markets draw the line in different places, so the same stone can change name and price by crossing a border.

Chromium is also what makes ruby fluoresce. Under ultraviolet light, and in strong sunlight, fine ruby glows red from within, which is part of why Burmese material with low iron content is so prized: iron quenches the fluorescence.

Rutile needles in three directions produce asterism, the six-rayed star seen in star rubies cut as cabochons.

Corundum has no cleavage but does have parting planes, and it is otherwise exceptionally durable.

Where it comes from

The Mogok valley in Myanmar is the classic source, where limestone containing traces of chromium was metamorphosed into ruby-bearing marble. Mogok material is low in iron, which is why it fluoresces strongly and shows the colour the trade calls pigeon's blood.

Mozambique became a major source from around 2009 and now supplies much of the market. Thailand and Cambodia produced iron-rich darker material and Bangkok remains a cutting and trading centre. Sri Lanka and Madagascar also supply the trade.

India's ancient trade ran through Sri Lanka and Burma, and Sanskrit texts describe ruby in detail. Burmese tradition connected rubies with invulnerability in battle, in the strongest form by inserting a stone beneath the skin, which is recorded here as history rather than repeated as advice.

What the traditions claimed

Sanskrit sources call ruby ratnaraj, the king of gems, and in the navaratna it answers to Surya, the Sun. It is prescribed for authority, vitality, the father and public standing.

The prescription is specific in a way no Western birthstone list is. An astrologer reads the strength and placement of the Sun in the chart, and only then decides whether ruby is appropriate. Weight is specified in ratti, metal is usually gold, the finger is the ring finger of the right hand, and the day and hour of first wearing are chosen. An astrologer may advise against it entirely.

Medieval European lapidaries give ruby the power to warn of coming danger by darkening, and to preserve the wearer's health, estate and, in Marbode's version, banish sorrow.

Burmese and Indian sources treat it as the stone of kingship, which is consistent with the Sanskrit name.

How it is worked with

If you want the tradition rather than the jewellery, the Jyotish route is the substantive one and it requires an actual chart reading. That is a different exercise from buying a July birthstone.

For wear, untreated or simply heated ruby is one of the best stones available: hardness 9, excellent toughness, and it can be worn in a ring indefinitely.

The sovereignty reading suits questions about authority you hold rather than authority you want: whether you are exercising it, whether you are avoiding it, and what it costs.

Red garnet and red spinel are the traditional substitutes in the Vedic system and are excellent stones in their own right, with spinel in particular having spent centuries mistaken for ruby in European crown jewels.

Buying it honestly

Ruby has the largest treatment and fraud problem of any gem.

Heat treatment is near-universal, permanent, stable and accepted when disclosed. Untreated fine material commands very large premiums and comes with laboratory reports.

Beryllium diffusion pushes colour into a surface layer at high temperature and must be disclosed, since a re-polish can remove it.

Glass filling is the serious one, and it has a date. From around 2004 the market absorbed large volumes of heavily fractured, low-grade rough flooded with high-refractive-index lead glass, which turns near-opaque material into something that looks like a clean transparent ruby. The filler is physically unstable: a jeweller's torch, an ultrasonic cleaner or an acidic household cleaner will damage it. Under magnification it shows gas bubbles and a blue flash along the fractures.

The trade's answer is terminology. The World Jewellery Confederation requires this material to be described as composite ruby or as a manufactured product rather than as ruby, and the Federal Trade Commission requires the treatment to be disclosed. Selling it as untreated ruby is not a grey area.

Synthetic ruby has been produced since Verneuil's flame-fusion process in 1902, in enormous quantity, and curved growth banding usually identifies it. Most inexpensive red stones in mass-market jewellery are synthetic.

One recent caution is worth carrying into a shop. Some Mozambican material has been irradiated to produce purple hues, and that colour is not stable: it fades on exposure to light. Irradiation is routine and permanent in blue topaz, and that does not make it permanent everywhere. Ask specifically whether a purple or unusually coloured corundum has been irradiated.